The human intestine contains billions of micro-organisms. The human body is composed of 10 times more microbial cells than of human cells.
When micro-organisms in our intestines meet a diet rich in carbohydrates and sugars, this changes the microbial environment and results in weight gain.
According to Transnational Science Medicine, researchers did experiments on mice. Initially, they took mice and removed the bacteria, and intestines were implanted with human microbes.
With a diet rich in fat and simple sugars, they noted a sharp change in the microbial community and the mice became obese.
As it would be easy to blame the fat, they also tested on mice with a diet low in fat and it gave the same result: the rodents gained weight.